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Division of Agricultural Sciences and Natural Resources • Oklahoma State University
F-7620
Oklahoma Cooperative Extension Fact Sheets
are also available on our website at:
http://www.osuextra.com
Kenneth E. Conway
Research Plant Pathologist
Brian Olson
Plant Disease Diagnostician
One of the greatest losses that a homeowner can ex-perience
is the loss of a tree. Not only is a tree an aesthetic part
of the yard but it also provides shade for the homeowner as
well as shelter for birds and other animals. Trees in the
landscape are often focal points for the entire family to enjoy
the wonders of nature. Therefore, it is tragic that, although it
takes years for a tree to develop, it requires only a short period
of time for a disease or diseases to kill it. Such is the case with
hypoxylon canker of oaks.
Occurrence
Although there were early records of hypoxylon canker of
oaks in Oklahoma, not much attention was given to its poten-tial
until the spring of 1979. Reports on the seriousness of this
disease were first received from Pushmataha County in
southeastern Oklahoma. A survey of several counties, as well
as specimens received in the Plant Disease Diagnostic Labo-ratory,
disclosed that the disease was very active in at least 14
counties in the eastern and central portions of the state.
The hypoxylon canker fungus infects most species of
oaks in many states and has been diagnosed from several
habitats, including forest sites, trees in pastures, recently
developed home sites, and established residential areas. In
one case in Oklahoma, the severity of the disease eliminated
15 acres of established oak trees surrounding a recently built
home. In another case, it eliminated six acres of oaks in the
central area of a condominium development.
Disease Cycle
The causal organism of oak hypoxylon canker is a fun-gus,
Hypoxylon atropunctatum. Unfortunately, little is known
about how this organism attacks and kills trees. However, it is
known that trees that have been stressed or weakened by
drought or have had their root systems injured are much more
susceptible to this disease than healthy trees.
Research reports indicate that the organism enters
branches through wounds. The fungus then grows through
the wounds, then through the sapwood causing decay. The
first outward symptoms that may be evident are yellowing and
wilting of leaves and death of top branches. Inoculation
experiments in Georgia have shown that the fungus is capable
of spreading up to 3 feet above and below a point of inoculation
within one growing season. Researchers at the University of
Arkansas have been able to isolate the fungus from seedling
oak trees which showed no symptoms of infection. This would
indicate that the fungus may be active in the trees for a number
of years before disease symptoms are noticed. When trees
are weakened, particularly by drought, or injured, the disease
is capable of overcoming resistance of the host, and the tree
dies.
The disease progresses through branches, causing die-back
symptoms. After the death of branches or of the tree, the
outer bark sloughs off, exposing a thin stroma (a mass or mat
of fungal hyphae packed together to form a hard crust in or on
which spores are formed). At first the stroma produces brown-ish,
dusty masses of conidia (asexual fungus spores) that are
easily blown from tree to tree and which cause new infections.
The stroma color soon changes to silver and then to black as
the sexual state of the fungus develops (Figure 1). The stroma
becomes thicker and harder as the sexual state develops. The
sexual state produces masses of dark spores inside the
stroma, which are “oozed” out onto the surface, where they
can be transferred by various means (rain, insects, etc.) to
other branches or trees. A large tree may be killed within one
to two years, depending on the vigor of the tree; however,
because early stages of the disease may not be noticed, trees
may appear to die within a period of a few weeks. A stroma
may be limited in its development or may extend the entire
length of the tree (Figure 2).
Control
There is no effective control for this disease, which is due
in part to lack of recognition of the early stages of infection. In
a commercial operation, where trees can be harvested for
pulp, it is recommended that trees be cut before decay
reduces their value.
In a home setting, individual trees that have more than
15% of the crown area infected should be cut to ground level
and burned. No stump should be left because stroma devel-opment
has been observed even on very small stumps. Trees
with less damage should be given extra care, such as water-ing
during periods of drought, providing adequate fertilization
and preventing damage to trunks or roots. Homeowners
should be aware that the fungus remains active on dead
wood. Therefore, if the wood is to be used as fuel, it should be
burned as soon as possible to prevent further spore produc-tion
and subsequent spread of disease. In addition, all dead
Hypoxylon Canker
of Oaks

Division of Agricultural Sciences and Natural Resources • Oklahoma State University
F-7620
Oklahoma Cooperative Extension Fact Sheets
are also available on our website at:
http://www.osuextra.com
Kenneth E. Conway
Research Plant Pathologist
Brian Olson
Plant Disease Diagnostician
One of the greatest losses that a homeowner can ex-perience
is the loss of a tree. Not only is a tree an aesthetic part
of the yard but it also provides shade for the homeowner as
well as shelter for birds and other animals. Trees in the
landscape are often focal points for the entire family to enjoy
the wonders of nature. Therefore, it is tragic that, although it
takes years for a tree to develop, it requires only a short period
of time for a disease or diseases to kill it. Such is the case with
hypoxylon canker of oaks.
Occurrence
Although there were early records of hypoxylon canker of
oaks in Oklahoma, not much attention was given to its poten-tial
until the spring of 1979. Reports on the seriousness of this
disease were first received from Pushmataha County in
southeastern Oklahoma. A survey of several counties, as well
as specimens received in the Plant Disease Diagnostic Labo-ratory,
disclosed that the disease was very active in at least 14
counties in the eastern and central portions of the state.
The hypoxylon canker fungus infects most species of
oaks in many states and has been diagnosed from several
habitats, including forest sites, trees in pastures, recently
developed home sites, and established residential areas. In
one case in Oklahoma, the severity of the disease eliminated
15 acres of established oak trees surrounding a recently built
home. In another case, it eliminated six acres of oaks in the
central area of a condominium development.
Disease Cycle
The causal organism of oak hypoxylon canker is a fun-gus,
Hypoxylon atropunctatum. Unfortunately, little is known
about how this organism attacks and kills trees. However, it is
known that trees that have been stressed or weakened by
drought or have had their root systems injured are much more
susceptible to this disease than healthy trees.
Research reports indicate that the organism enters
branches through wounds. The fungus then grows through
the wounds, then through the sapwood causing decay. The
first outward symptoms that may be evident are yellowing and
wilting of leaves and death of top branches. Inoculation
experiments in Georgia have shown that the fungus is capable
of spreading up to 3 feet above and below a point of inoculation
within one growing season. Researchers at the University of
Arkansas have been able to isolate the fungus from seedling
oak trees which showed no symptoms of infection. This would
indicate that the fungus may be active in the trees for a number
of years before disease symptoms are noticed. When trees
are weakened, particularly by drought, or injured, the disease
is capable of overcoming resistance of the host, and the tree
dies.
The disease progresses through branches, causing die-back
symptoms. After the death of branches or of the tree, the
outer bark sloughs off, exposing a thin stroma (a mass or mat
of fungal hyphae packed together to form a hard crust in or on
which spores are formed). At first the stroma produces brown-ish,
dusty masses of conidia (asexual fungus spores) that are
easily blown from tree to tree and which cause new infections.
The stroma color soon changes to silver and then to black as
the sexual state of the fungus develops (Figure 1). The stroma
becomes thicker and harder as the sexual state develops. The
sexual state produces masses of dark spores inside the
stroma, which are “oozed” out onto the surface, where they
can be transferred by various means (rain, insects, etc.) to
other branches or trees. A large tree may be killed within one
to two years, depending on the vigor of the tree; however,
because early stages of the disease may not be noticed, trees
may appear to die within a period of a few weeks. A stroma
may be limited in its development or may extend the entire
length of the tree (Figure 2).
Control
There is no effective control for this disease, which is due
in part to lack of recognition of the early stages of infection. In
a commercial operation, where trees can be harvested for
pulp, it is recommended that trees be cut before decay
reduces their value.
In a home setting, individual trees that have more than
15% of the crown area infected should be cut to ground level
and burned. No stump should be left because stroma devel-opment
has been observed even on very small stumps. Trees
with less damage should be given extra care, such as water-ing
during periods of drought, providing adequate fertilization
and preventing damage to trunks or roots. Homeowners
should be aware that the fungus remains active on dead
wood. Therefore, if the wood is to be used as fuel, it should be
burned as soon as possible to prevent further spore produc-tion
and subsequent spread of disease. In addition, all dead
Hypoxylon Canker
of Oaks